Lecture - Ann Arbor Earth Science


Lecture - Ann Arbor Earth Science

Unit 5: Age of the Earth


E5.3D Describe how index fossils can be used to determine time


E5.3g Identify a sequence of geologic events using relative-age

dating principles.

Relative Dating

In the same way that a history book shows an order of events, layers of rock

(called strata) show the sequence of events that took place in the past. Using a

few basic principles, scientists can determine the order in which rock layers

formed. Once they can know the order, a relative age can be determined for each

rock layer.

*Relative age indicates that one layer is older or younger than another layer, but

does not indicate the rock’s age in years (absolute age).

Law of Superposition

Sedimentary rocks form when new

sediments are deposited on top of old

layers of sediment. As the sediments

accumulate, they are compressed and

harden into sedimentary rock layers.

Scientists use a basic principle called the

Law of Superposition to determine the

relative age of a layer of sedimentary

rock. *The Law of Superposition is

that an undeformed sedimentary rock

layer is older than the layers about it and

younger than the layers below it.

According to the Law of Superposition,

layer 1 was the first layer deposited, and

thus the oldest layer. The last layer

deposited was layer 12, and thus it is the

youngest layer.

*Principle of Original


Scientists also know that sedimentary rock generally forms in

horizontal layers. The Principle of Original Horizontality is

that sedimentary rocks left undisturbed will remain in horizontal

layers. Therefore, scientists can assume that sedimentary rock

layers that are not horizontal have been tilted or deformed by

crustal movements that happened after the layers formed.


Movements of Earth’s crust can lift up rock layers

that were buried and expose them to erosion.

Then, if sediments are deposited, new rock layers

form in place of the eroded layers. The missing

rock layers create a break in the geologic record

in the same way that pages missing from a book

create a break in a story.

*A break in the geologic record is called an

unconformity. An unconformity shows that

deposition stopped for a period of time, and rock

may have been removed by erosion before

deposition resumed.


There are three types of unconformities.

1. An unconformity in which stratified (layers) of

rock rests upon unstratified rock is called a


2. The boundary between a set of tilted layers

and a set of horizontal layers is called an

angular unconformity.

3. The boundary between horizontal layers of old

sedimentary rock and younger, overlying

layers that are deposited on an eroded surface

is called a disconformity.

According to the Law of Superposition, all rocks beneath an unconformity are

older than the rocks above the unconformity.

Crosscutting Relationships

When rock layers have been disturbed by faults (a break or crack in Earth’s crust)

or intrusions (a mass of igneous rock that forms when magma is injected into rock

and then cools and solidifies), determining relative age may be difficult. In such

cases, scientists may apply the Law of Crosscutting Relationships. *The Law

of Crosscutting Relationships is that a fault or intrusion is always younger

than all the rocks it cuts through above and below the unconformity.

Relative Age

Based on what you now know about the Law of

Superposition, the Principle of Original

Horizontality, unconformities, and the Law of

Crosscutting Relationships can you place the layers

indicated in the diagram in the correct order,

starting from the oldest layer?

The oldest layer is Q, followed by O, then N, then

M, then L. P cuts across layers L-Q, so it is the

next layer since it does not cut into layer H.

Above the unconformity we then have layer H,

followed by I, then J, with K being the youngest


*Index Fossils

Paleontologists can use fossils to determine the relative ages of the rock layers in

which the fossils are located. Fossils that occur only in rock layers of a particular

geologic age are called index fossils.

To be an index fossil, a fossil must meet certain requirements:

1. It must be present in rocks scattered over a large region.

2. It must have features that clearly distinguish it from other fossils.

3. Organisms from which the fossil formed must have lived during a short span of

geologic time.

4. The fossil must occur in fairly large numbers within the rock layers.

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